The present invention relates generally to light valves and more particularly to the use of liquid crystal light valves for converting visible wavelength images to infrared wavelength images.
In the development of infrared imagers or "seekers" for advanced search and tracking systems, it is important to be able to test the infrared imagers with the same complex, dynamic scenes as they will encounter under actual field conditions. It is also important to be able to do so non-destructively to minimize cost and time factors. Therefore, computerized simulation systems are used for non-destructive infrared imager testing. Various field tests can be simulated and projected to the infrared imager for performing parametric testing in the laboratory. The advantages of such simulation systems are that they facilitate mission variation, accurately reproduce test environments, and increase proficiency in implementing improvements.
Computerized simulation systems generate an infrared coded visible wavelength image on a cathode ray tube (CRT) and convert it to a high-power infrared image which is projected onto an infrared imager under test. Both the CRT and the infrared imager are coupled to a computer which generates the visible images and monitors infrared images "seen" by the infrared imager. The heart of the simulator is an image converter which takes the infrared coded visible images on the CRT and converts them to the infrared images sensed by the infrared imager.
Ideally, a simulation system should be capable of providing a complex, fast-changing infrared scene with a high degree of dynamic range and resolution, and likewise be capable of real-time interaction with the computerized image generation system. Existing simulation systems have difficulty in meeting these criteria, primarily because of the prior IR conversion devices which have been used.
These prior devices typically utilize thermally produced radiation from thin-film materials for image conversion. Operation of such devices consists of illuminating the film with a high intensity visible beam containing scenery information from a CRT (or movie projector). The film heats up locally in response to the image intensity of the input visible beam and then emits an infrared wavelength pattern that is related to the spatial information presented by the high intensity visible input beam. With such devices, thermal images are projected directly onto the infrared imagers, and the input visible source is directly coupled to the output infrared image, both of which are disadvantageous.
In addition, the thin film infrared simulation systems suffer from low resolution caused by thermal spread in the image plane; slow thermal response caused by large thermal time constants; and limited dynamic range due to the conflict between higher resolution and the dynamic range of a thermal based simulation system.
Liquid crystal-based "light valves" or spatial light modulators have been proposed for use as infrared image converters due to their ability to modulate an infrared beam by polarization-rotation. An example of such a device is set forth in U.S. Pat. No. 4,114,991, issued to W. T. Bleha on Sept. 19, 1978. The development of, and theory underlying, light valve technology is illustrated in such patents as U.S. Pat. No. 3,824,002, issued to T. D. Beard on July 16, 1974 and U.S. Pat. No. 4,019,807, issued to D. D. Boswell, et al. on Apr. 26, 1977. The basic design of alternatingcurrent (AC) light valves is shown in the Beard patent. The configuration and operation of the light valve in the hybrid field effect mode, which accomplished the polarization-rotation necessary to effect spatial modulation of an infrared beam is discussed in the Boswell Patent, as well as in U.S. Pat. No. 4,378,955, issued to W. T. Bleha on Apr. 5, 1983. All of these patents are owned by the assignee of the present invention.
Two primary problems exist with employing existing light valves in the infrared wavelength spectral region which relate to the electro-optical properties of the liquid crystals: speed and contrast. The response time (either write-time or decay-time) of the liquid crystal electro-optic effect is inversely proportional to the square of the thickness of the liquid crystal layer. Conversely, contrast is a function of the birefringence of the liquid crystal and the thickness of the liquid crystal. The birefringence and efficiency of the liquid crystal material decrease with increasing wavelength. Thicker liquid crystals are, therefore, required at longer wavelengths to maintain the same level of contrast. Accordingly, the inherent problem with existing light valves used for infrared image conversion is that the thicker liquid crystal layer required for contrast purposes simultaneously results in a slower response time which is unsuitable for the fast real-time infrared conversion needed to test advanced infrared imagers.
Accordingly, it is the principal purpose of the present invention to provide a high performance, non-thermal dynamic visible to infrared image converter.
Another purpose of the present invention is to achieve both a high contrast and a fast response time in a light valve used as a visible to infrared imager converter.
Yet another purpose of the present invention is to utilize a low-power image source in an infrared simulation system.